Optical waveguide

ABSTRACT

A waveguide, which is part of an integrated optical circuit is arranged onto a planar substrate, and has a core section propagating light to a direction of propagation. The waveguide is a conversion waveguide between a ridge-type waveguide and a strip waveguide. In the conversion waveguide, the core section is made of the one and same material so that the cross-section of the core section transverse to the direction of propagation of light is two-step from both edges. The conversion waveguide has two layers of different widths so that the height of the first layer is equal to the height of the ridge of the ridge-type waveguide, and the height of the second layer is equal to the height of the base part of the ridge-type waveguide.

The invention relates to an optical waveguide according to preamble ofclaim 1, the optical waveguide being part of an integrated opticalcircuit.

The invention also relates to a method according to preamble 5 formanufacturing an optical waveguide for an integrated optical circuit.

An integrated circuit consists of a set of optical circuit elements,devices and/or external connections, which are irremovably connected toeach other by optical waveguides and which are arranged onto a commonsupport. For example, light sources and detectors, power splitters,switches, wavelength splitters and connectors, and fibre connections canbe circuit elements. They have been manufactured either by the same or adifferent method as the optical waveguides connecting them.

The optical waveguide refers next to a three-dimensional structurearranged onto a planar support, substrate, which transfers light fromone place to another in an integrated circuit. The direction of theoptical waveguide in the plane of the support can be constant, or it canchange either in a slowly curving or suddenly turning manner. Thecross-section of the optical waveguide can be either constant, or it canchange slowly or suddenly. There are often several such differentoptical waveguide cycles sequentially. The basic material of the supportis, for example, silicon, compound semiconductor or glass. The materialof the optical waveguide can be, for example, silicon, semiconductorcompound, glass or organic substance.

The optical waveguide has a certain three-dimensional refractive indexdistribution n(x, y, z), which together with material attenuationdetermines how light with a certain wavelength λ travels in the opticalwaveguide and what are its propagation losses. The used wavelength λ oflight, generally optical radiation, extends from the area of visiblelight to near-by infrared area.

The cross-section of the optical waveguide is examined in a plane, whichis perpendicular to the direction of propagation of light, the z axis ordirection. The cross-section of a straight optical waveguide isconstant, and its refractive index distribution n(x, y) is substantiallytwo-dimensional. On the basis of the cross-section, it is theoreticallypossible to calculate the number of discrete propagating modes in astraight optical waveguide, the effective refractive indexes andtransverse field distributions. The calculations are usually madenumerically, as no analytic solution is generally not available. Theeffective refractive index describes the propagation speed of lightconnected to the mode along the optical waveguide in a similar way asthe refractive index of the material describes the propagation speed ofan optical plane wave in it. The modes can generally be divided into twogroups according to their polarisation, the difference of which dependson the asymmetry of the optical waveguide and/or the birefringence ofthe materials. For simplifying the description, only modes of theso-called TE (quasi transverse electric) type are examined next, but allprinciples also apply to other polarisation modes, such as modes of theTM (quasi transverse magnetic) type.

A special case of a straight optical waveguide is a so-called planarwaveguide conductor, which has not been patterned in any way in thedirection horizontal to the surface of the support, i.e. the xdirection. The refractive index distribution n(y) of the planarwaveguide conductor is substantially unidimensional, and it correspondseither to an infinitely wide or narrow straight optical waveguide. Thenumber of discrete propagating modes, effective refractive indexes andvertical field distributions can be calculated for the planar waveguideconductor with the same principle as for finitely wide opticalwaveguides, but more simply.

The propagation of modes in a straight optical waveguide is based on thetotal reflection of light between the core area of the optical waveguideand the areas surrounding it both in horizontal and vertical direction.This requires that the refractive index of the core area is higher thanthe refractive index of the materials surrounding it. When a ridge-typewaveguide or a similar structure is concerned, a so-called effectiverefractive index difference can prevail in the place of the refractiveindex difference of the materials in either direction. In practice, alsospecific attenuations of the materials and the scattering of light fromnon-ideal material interfaces also influence the propagation. Inaddition to propagating modes, an infinite number of so-called radiationmodes can be calculated for a straight optical waveguide, to which nototal reflection is connected at least at all edges of the core area.The arbitrary optical field distribution connected to the straightoptical waveguide can be unambiguously presented as a weighted sum ofpropagating and radiation modes. The power connected to radiation modesgradually radiates away from the optical waveguide.

Other than straight optical waveguides do not generally have suchpropagating modes, the transverse power distribution of which remainsunchanged, and which do not continuously radiate power away from thecore area. On the basis of cylinder symmetry, discrete modes propagatingin a curving manner can be calculated for optical waveguides curvingwith a constant radius, but the finitely attenuating field distributionsof all curving optical waveguides forcibly radiate power to thedirection of the outer curve/see reference publications 1, 2/. Also theoperation of other optical waveguides besides straight ones can bepresented with the help of modes, but the number of modes and effectiverefractive indexes calculated for them, the field distributions of themodes and/or the shares of the total power of the optical waveguide indifferent modes change. As the cross-section changes or the direction ofthe optical waveguide changes, optical power is usually connected fromone mode to the other. In so-called adiabatic optical waveguidestructures changing sufficiently slowly in the direction of propagation,however, power is never transferred from one mode to another, but thepower stays in the same optical waveguide mode slowly changing its fielddistribution.

It has been tried to arrange the cross-section of a straight opticalwaveguide so that it allows at the least the so-called fundamental mode,the mode number m of which is 0, to propagate in the optical waveguideand, most preferably, with as small losses as possible. An opticalwaveguide with only one propagating mode (m=0) is called a single-moded(SM) optical waveguide. An optical waveguide with more than onepropagating mode (the mode numbers m=0, 1, 2, . . . ) is called amulti-moded (MM) optical waveguide. Multi-modedness does not necessarilymean that power is transferred from the fundamental mode to higher modesin the optical waveguide. Examined externally, the single-moded opticalwaveguide connection can consist, for example, of single-moded opticalwaveguide sequences and multi-moded, but adiabatic optical waveguidesequences between them/reference publication 2/. Especially intelecommunications technology, integrated optical circuits generallyhave to operate single-modedly, when examined externally. More complexintegrated optical circuit elements (power splitters, etc.) oftenconsist of multi-moded optical waveguide structures even in single-modedsystems.

A straight optical waveguide 1; 4 is previously known, which is arrangedonto a planar support 2, as is illustrated in FIGS. 1 and 2. The opticalwaveguide has a projection 1 ¹; 4 ¹ patterned to the core material andconveying light to a certain linear direction. The side edges of theprojection need not necessarily be vertical, but they can also be, forexample, oblique or rounded. Alternatively, there is provided one orseveral material layers between the projection and the support. On topof and at the sides of the projection there can respectively be providedone or several surface material layers 3. The material layers can eitherconsist of solid, liquid or gaseous material. However, the refractiveindex distribution of the cross-section of the optical waveguide isalways such that it makes possible the existence of at least onepropagating mode. Only those material layers and areas, to which theoptical power distribution of at least one propagating mode extends, aregenerally included in the theoretical optical waveguide structure, bothin the horizontal and vertical direction. At the same time, theoutermost material layers and areas are assumed to extend infinitely faraway.

In one known optical waveguide 1, FIG. 1, the refractive index of theprojection 1 ¹ is bigger than the refractive indexes of the surroundingmaterials. Irrespective of the form of the side edges, such an opticalwaveguide is in the following called a rectangular optical waveguide. Init light usually total reflects on the horizontal and vertical surfaceslimiting the projection. If there are several material layers above orbelow the projection, the total reflection can alternatively occur onlyon some outer interface. If the layers above and below the rectangularoptical waveguide have the same or at least almost the same refractiveindexes, and if its side edges are vertical, the optical waveguidestructure is symmetrical, besides the horizontal direction, also in thevertical direction. In this case, also the field distribution of thefundamental mode of the rectangular optical waveguide is symmetrical inthe vertical direction.

In a second known optical waveguide 4, FIG. 2, the projection 4 ¹ isseamlessly arranged onto a thin unpatterned layer of the same corematerial, i.e. the base element 4 ². The projection 41 and the baseelement 42 form the ridge-type optical waveguide 4. In the ridge-typeoptical waveguide, the vertical total reflection occurs on thehorizontal material interfaces following the same principle as in therectangular optical waveguide. However, the horizontal total reflectionis based on the so-called effective refractive index difference /seereference publication 1/. In the case of vertical side walls, theapproximative effective refractive index difference is obtained bycomparing the effective refractive indexes calculated from the verticalunidimensional refractive index distributions at the place of andadjacent to the projection. However, for an exact optical waveguideanalysis it is necessary to resort to two-dimensional numerical methods.The refractive index structure of the ridge-type optical waveguide isasymmetrical in the vertical direction, and because of this, also thefield distribution of its fundamental mode is asymmetrical in thevertical direction. As the effective refractive index difference of theridge-type optical waveguide decreases, for example, upon narrowing orlowering the projection, the asymmetry of its field distributionincreases simultaneously in the vertical direction.

The basis in a known method, the etching method, for manufacturing oneor several optical waveguides to be arranged to an optical integratedcircuit is a planar support, onto which an initially unpatterned corelayer of the optical waveguide is prearranged, as well as one or severalmaterial layers. The topmost layer, the so-called resist layer, ispatterned by one or several known alternative methods so that aso-called process pattern is reproduced to it as a resist mask. Knownresist patterning methods are presented below. The process patternrefers to a two-dimensional pattern which determines from which areas ofthe support the resist will be removed and to which areas it will beleft. At its simplest, the light controlling structure is a straightoptical waveguide, in which the process pattern comprises a line ofconstant width. Generally one process pattern nevertheless presents alloptical waveguide structures to be processed to one support. In theetching phase, the structure on the support is etched by using one ofthe several known alternative methods so that the patterned resist maskprotects the parts of the material layer or layers beneath it, and theprocess pattern is thus reproduced to the said layers. Known etchingmethods are, among others, wet and dry etching. A preferable dry etchingmethod is ICP (inductively coupled plasma) etching. If there is one orseveral so-called hard mask layers between the resist and core material,the etching of the structure is performed in several different phases.In this case, the pattern of the resist mask is first produced to thefirst hard mask layer by etching. The generated structure can then beused as a new mask in the etching of the next hard mask layer, and soon. After the patterning of the last hard mask layer, the projection isfinally patterned to the core material layer by etching. Between andafter the etchings, upper resist or hard mask layers can be removed bymaterial selective intermediate etching phases. After the patterning ofthe projection, surface material layers can still be grown or otherwiseformed on top of and to the sides of the projection.

Known resist patterning methods are, among others, optical lithography,electron beam lithography, phase mask lithography, and mechanicalimprint technology. The use of the preferable optical lithography in theetching method is next described in more detail. In optical lithography,the process pattern is first formed as a patterned metal layer, i.e. asa so-called exposure mask, to the surface of a separate glass plate. Alight-sensitive, such as an ultraviolet light sensitive material is usedas the resist layer. A certain section of the surface of the support iscovered by the metal patterns of the exposure mask, and the uncoveredsections of the surface are exposed to UV light. In the development ofthe resist, the resist is removed either from the exposed or unexposedareas, depending on the resist process used.

The basis in a second known method, the growing method, in themanufacture of one or several optical waveguides to be arranged to anoptical integrated circuit is a support, onto which an optical waveguidewith the desired properties is grown of one or several materials. In thegrowing method, a structure controlling the growing is usually formed tothe support by using the etching method before the growing, thestructure directing the growing of new material layers only to thedesired places.

A drawback in the above mentioned known optical waveguides and theirmanufacturing methods is the deficiency in their versatility. In all ofthem, only one process pattern is used for determining one opticalwaveguide, in which case the width of the optical waveguide, theeffective refractory index difference, the number of modes and thesymmetry/asymmetry of the field distribution cannot be freely determinedseparately.

The object of the invention is to eliminate the drawbacks related to theabove disclosed optical waveguides applicable to integrated optics. Theobject of the invention is also to achieve a new optical waveguide and anew method for its manufacture.

The optical waveguide of the invention is characterised in what isdisclosed in claim 1.

The method of the invention for manufacturing an optical waveguide of anintegrated optical circuit is characterised in what is disclosed inclaim 5.

The dependent claims disclose advantageous embodiments of the invention.

The optical waveguide according to the invention is part of anintegrated optical circuit, the optical waveguide being arranged onto aplanar support and including a core element conveying light to a certaindirection, the direction of propagation.

In accordance with the invention, the optical waveguide is a modifiedoptical waveguide between the ridge-type optical waveguide and therectangular optical waveguide, the core element in the modified opticalwaveguide being manufactured of the one and same material so that itscross-section transverse to the direction of propagation of light istwo-stepped on both sides, and the modified optical waveguide containingtwo layers of different widths, the height of the first layer beingequal to the height of the ridge in the ridge-type optical waveguide,and the height of the second layer being equal to the height of the basesection of the ridge-type optical waveguide, the sum of the heights ofthe layers being equal to the height of the rectangular opticalwaveguide and the widths of the two layers being arranged to changeuniformly between the optical waveguides to be connected for fittingthem in the lateral direction.

The core section of the optical waveguide according to the inventionforms a projection in relation to the support, the both longitudinalsides of which consist of two steps, each single step being providedwith an ascending wall and a stair plane, respectively. The steps arethen formed of alternately repeating ascending walls and stair planes.It has to be noted that the ascending walls are not necessarilyvertical, but they can be, for example, oblique or rounded.Respectively, the stair planes of the steps are not necessarilystraight, especially horizontal, planes, because also they can beoblique and/or rounded. However, adjacent steps are separatelyidentifiable, and their location is determined either on the basis ofdifferent process patterns or different process pattern combinations.

The optical waveguide of the invention is most preferably made onto asemiconductor support, especially a silicon wafer. The optical waveguideis processed onto a planar support and especially onto a light-conveyingcore layer on top of it, most preferably by a method of the invention.

The advantage of the invention is that with it it is possible toadiabatically change the type of the optical waveguide from a ridge-typeoptical waveguide to a rectangular optical waveguide. In structures witha large refractory index difference and coarsely identical dimensionslarger than the wavelength, the ridge-type optical waveguide can besingle-moded and the respective rectangular optical waveguide againclearly multi-moded. Because of the invention, simple variations fromsingle-moded waveguides to multi-moded waveguides are possible both inthe vertical and horizontal direction.

The advantage of the optical waveguide of the invention also is thatwith the help of it, the small effective refractory index difference ofthe ridge-type optical waveguide can be adiabatically changed to thevery large effective refractory index difference of the rectangularoptical waveguide.

The rectangular optical waveguides with a large effective refractoryindex difference have considerable advantages, compared with theridge-type optical waveguides with a small effective refractory indexdifference. Using them it is, for example, possible to provide verysmall optical waveguide curves with small losses, and so-called opticalwaveguide mirrors steeply changing the direction of light and based onthe total reflection of light. With them it is also possible to provideconsiderably more propagating, especially horizontal modes to an opticalwaveguide of a certain width. This large number of horizontal modes canbe utilised, for example, for reducing the size of so-called multi-modeinterference couplers (MMI couplers), which are based on the controlledinterference between horizontal modes. The length of an MMI couplergrows generally quadratically in relation to the width of an MMI opticalwaveguide, and the minimum of the width is again determined on the basisof the minimum number of required modes. In the rectangular opticalwaveguide, the large number of modes makes it possible to use clearlynarrower MMI optical waveguides so that the length of the MMI couplercan be considerably shortened. When switching light to the MMI opticalwaveguide and away from it, it is however always necessary to make surethat light power is not coupled to the modes higher in the verticaldirection at any stage.

An advantage of the optical waveguide of the invention is also that withit components based on rectangular optical waveguides can beadiabatically connected between single-moded ridge-type opticalwaveguides, such as small-sized optical waveguide curves, opticalwaveguide mirrors and short MMI couplers. Such optical waveguideconnections can operate externally single-moded.

An advantage of the optical waveguide of the invention is also that inthe vertical direction, i.e. in relation to the horizontal plane, theasymmetric field distribution of a ridge-type optical waveguide can bechanged to a field distribution of a rectangular optical waveguide,symmetric in the vertical direction. Vertical symmetry can be utilised,among others, for decreasing the attenuation of MMI couplers and/or forreducing their size. As has been stated above, a rectangular opticalwaveguide is better suitable for providing short MMI couplers than aridge-type optical waveguide. If a ridge-type optical waveguide isdirectly connected to the a rectangular MMI optical waveguide, thevertical asymmetry of the ridge-type optical waveguide and the verticalsymmetry of the rectangular optical waveguide cause between them adetrimental coupling of light power to modes higher in the verticaldirection. By using the modified optical waveguide of the inventionbetween the ridge-type optical waveguide and the rectangular MMI opticalwaveguide, small MMI couplers can be connected to the ridge-type opticalwaveguides without the coupling problem mentioned above.

The method of the invention is directed to the manufacture of an opticalwaveguide of an integrated optical circuit onto a support. According tothe invention, the optical waveguide is a modified optical waveguide,which is manufactured between the ridge-type and rectangular opticalwaveguides onto such a planar support, on which there is provided alight conveying core layer, in which method the core layer iscontrollably thinned in two phases for forming two different steps onboth sides of the modified optical waveguide so that during the twothinning phases a different process pattern is utilised, the edges ofwhich determine the location of the step edges of the optical waveguideon the support so that the result is a two-step optical waveguidestructure from both sides in the direction transverse to the directionof propagation of light, and in which the modified optical waveguide isprovided with two layers of different widths, the height of the firstlayer being arranged equal to the height of the ridge in the ridge-typeoptical waveguide and the height of the second layer being arrangedequal to the base part of the ridge-type optical waveguide, and in whichthe sum of the heights of the layers is arranged equal to the height ofthe rectangular optical waveguide, and the widths of the two layers arearranged to change uniformly between the optical waveguides to beconnected for fitting them in the lateral direction. The ridge-typeoptical waveguide and the rectancular optical waveguide are bothdetermined with the help of one process pattern only. However, theoptical waveguide of the invention, i.e. the modified optical waveguideis determined with the help of the combination of two different processpatterns.

An advantage of the method of the invention is that with it theridge-type optical waveguide can be adiabatically changed to therectangular optical waveguide in a reliable and easy way and with smallpower losses.

An advantage of the method of the invention is also that it is notespecially sensitive to centering errors occurring between differentprocess patterns.

The invention and its other advantages are next explained in moredetail, referring to the enclosed drawings, in which

FIG. 1 is a cross-section of a first optical waveguide according to thestate of the art, i.e. a rectangular optical waveguide;

FIG. 2 is a cross-section of a second optical waveguide according to thestate of the art, i.e. a ridge-type optical waveguide;

FIG. 3 is a cross-section of the support;

FIG. 4 is a block diagram of the method for manufacturing the opticalwaveguide in phases;

FIGS. 5A and 5B illustrate the manufacture of the optical waveguide andpresent two different phases of readiness;

FIG. 6 is a perspective view of the optical waveguide of the invention,with the help of which the ridge-type optical waveguide can be convertedto the rectangular optical waveguide, or vice versa; and

FIGS. 7A, 7B, 7C are cross-sections A-A, B-B and C-C of the opticalwaveguide in FIG. 6, respectively.

The invention relates to an optical waveguide, which is part of anoptical integrated circuit. The optical waveguide has a core elementconveying light to a certain direction, the direction of propagation.The optical waveguide, especially its two-step core element, is arrangedonto a planar support 7, FIG. 3. The refractive index of the layer ormaterial below the core element on the wavelength in question is smallerthan the corresponding refractive index of the core element. Forexample, a photolithographic method, FIG. 4, is used in the manufactureof the optical waveguide of the invention, the method being explained inmore detail later in this application.

In an advantageous embodiment of the invention, the common support ismost preferably a support made of semiconductor, such as a semiconductorwafer that is generally used also as a support for electronic integratedcircuits. The support works as a physical foundation, onto which anumber of integrated optical circuits are arranged.

The support 7 of the optical waveguide, FIG. 3, is preferably a SOI(silicon on insulator) wafer. The SOI wafer consists of a thick siliconwafer 7 a, on which there first is a thin silicon oxide layer 7 b, andon top of that a thin core layer 7 c of silicon. The oxide layer 7 bacts as a so-called buffer layer, which optically insulates the corelayer 7 c from the silicon wafer below, due to its refractory index,which is smaller than that of silicon. The thickness of the oxide layer7 b is typically 0.5-3 μm, but it can also be as much as 1-15 μm. Therefractory index of silicon is about n=3.5 and, respectively, therefractory index of silicon is about n_(a)=1.5, depending on thewavelength of light. The wavelength λ of the light used is about 1-2 μm,preferably, for example, 1.55 μm. FIG. 3 expressly presents a SOI wafer,but alternatively, also several different single- or multi-layerstructures can be used as the support. Instead of silicon, for example,gallium arsenide (GaAs) or other respective material can bealternatively used as the material for the core layer.

In the method of the invention, the optical waveguide 60, FIG. 6, ismanufactured onto a suitable finished support 7, FIG. 3, such as a SOIwafer, on which there already is a light conducting core layer 7 c. Inthe method of the invention, the optical waveguide 60, especially itscore element 600, is made so that the core layer 7 c on the support iscontrollably thinned in two different phases for forming the differentsteps 6; 6 ^(1a) 6 ^(2a) 6 ^(3a) 6 ^(1b) 6 ^(2b) 6 ^(3b) and the layers60 ¹, 60 ², a different process pattern being utilised in both thinningphases, the area dimensions of which, i.e. width and length, correspondto the area dimensions of the different layers of the optical waveguideso that the result obtained is an optical waveguide structure two-stepfrom both edges, transverse to the direction of propagation of light.Thus, the edges of the different process patterns determine the locationof the edges of the steps of the optical waveguide in the core layer 7 con top of the support. In the same connection, also other possibleintegrated optical waveguides related to the optical waveguide 60 areprepared.

The optical waveguide 60 of the invention is illustrated as aperspective view in FIG. 6, and its cross-sections are illustrated inFIGS. 7A, 7B and 7C. The optical waveguide 60 is a modified opticalwaveguide, which is arranged between the ridge-type optical waveguide 61and the rectangular optical waveguide 62, which are known as such.

In the optical waveguide 60 of the invention, there are two successiveand seamless material layers 60 ¹, 60 ² made of the same material, whichform the core element 600. The layers 60 ¹, 60 ² of the opticalwaveguide 60 have different widths 1 _(60a), 1 _(60b) so that the steps6; 6 ^(1a), 6 ^(2a); 6 ^(1b), 6 ^(2b) are formed to the edges 60 a, 60 bof the optical waveguide 60. The optical waveguide 60 can also besurrounded by a surface layer, i.e. shell (not shown in the figures).This shell can be made of a suitable solid material, which is added ontothe optical waveguide 60 in connection of the manufacture, or it can bea gaseous shell, such as surrounding air, or even a liquid shell. Theshell can also consist of more than one layer or material.

The optical waveguide 60 of the invention is made onto the planarsupport 7, as has been shown above. The two-step patterning of the corelayer required in the realisation can be made, for example, by using thephotolithographic manufacturing method described next, presented as ablock diagram in FIG. 4. Some manufacturing phases have been illustratedin FIGS. 5A and 5B. However, it has to be noted that the opticalwaveguide 60 of the invention can also be realised by many otheralternative methods.

As the optical waveguide 60 of the invention is manufactured using thephotolithographic manufacturing method, the support 7 is first taken, tothe core layer 7 c on top of which the optical waveguide is intended tobe arranged (phase 40). The support is a preprocessed wafer, forexample, a SOI wafer (cf. FIG. 3). In the first manufacturing phase 41,a hard mask layer 9; 9 ¹, such as a silicon dioxide layer, is added tothe surface of the wafer. In the second phase 42, a resist, i.e. alight-sensitive protective layer 10; 10 ¹ (cf. FIG. 5A) is added on topof the hard mask layer 9; 9 ¹. After this, in phase 43, the preprocessedwafer with the first process mask, i.e. in this case the exposure mask11; 11 ¹ is fitted to an exposure device, in which the support 7 and theprocess pattern 11 are located parallel to and at a small distance fromeach other, and they are exposed (cd. FIG. 5B). In this case, the light12, especially UV light, is let to affect the surface layers of thesupport and especially the light-sensitive protective layer 10; 10 ¹through the apertures 11 a ¹, 11 b ¹ of the exposure mask 11; 11 ¹.Thus, a picture of the exposure mask, especially its edges, is arrangedto the surface of the wafer. In the next fourth phase 44, the exposedwafer is developed so that the exposed parts of the light-sensitive filmare detached. After this, the wafer is etched in the fifth phase 45 sothat of the areas that became unexposed in the development, first thehard mask layer and then the first grooves 13, 14 can be etched to thedesired depth h₁. The etchings of the hard mask and the core layer aregenerally separate process phases, although they have been shown here asone phase for the sake of simplicity. After the etching, the firstprojection 15 remains between the grooves 13, 14, the height of theprojection being h₁ and the width 1 ₁. In the sixth phase 46 the resist10; 10 ¹ is removed. In the seventh phase 47, the hard mask layer 9; 9 ¹is removed from the unetched areas. Thus, the first processing cycle q=1has been performed, and it is possible to move to the second processingcycle q=q+1.

The second processing cycle begins principally in the same way as thefirst processing cycle: a new hard mask layer is first added onto thesupport already once processed, and the light-sensitive protective layeris also added onto the hard mask layer, i.e. the first and second phase41, 42 are performed again. After this one moves to the third phase 43,and the exposure with the second process mask, i.e. in this case, theexposure mask, is performed. The light is again let to influence thesurface layers of the support through the apertures of the secondexposure mask. In the fourth and fifth phase 44, 45, the support isagain developed and etched, as the result of which in this applicationexample, all the areas exposed during this second processing cycle areetched until the lower edge of the core layer. The resist is thenremoved in the sixth phase 46 and the hard mask layer in the seventhphase 47. Thus, also the second processing cycle q=2 has been performed,and the core layer of the optical waveguide 60 is etched to form atwo-step layer.

In the photolithographic manufacturing process described above, aseparate hard mask layer is presented to be added at the beginning ofboth processing cycles and, respectively, to be removed at the end ofthe same processing cycle. However, this is not always necessary, butthe same hard mask layer can be used in both processing cycles. In thiscase, the deepening of all the grooves already made is continued duringthe latter processing cycle, and the etching of new grooves is furtherinitiated.

The etching depths and the widths of the projections restricted by theetched grooves are typically between 0.5-15 μm with a support that is aSOI wafer.

In the manufacture of the optical waveguide of the invention describedabove, each material layer and the respective step of the opticalwaveguide were made successively, beginning from the uppermost layer andthe respective step 6 ^(1a), 6 ^(1b). However, the order of theprocessing cycles and, at the same time, the order of use of the processpatterns can be changed. It especially has to be noted that some areascan be overecthed so that the sum of the etching depths in these isbigger than the original thickness of the core layer. In this case, thepossible continuation of the etching to the layers below the core layerdepends on the materials of the layers in question and on the etchingmethod used.

The structure of the optical waveguide 60 of the invention is nextexplained in more detail referring to FIGS. 6, 7A, 7B, 7C.

The height of the uppermost layer of the optical waveguide 60, i.e. thefirst layer 60 ¹ and at the same time the rise h_(60a) of the uppermoststep is equal to the height h_(h) of the ridge 61 ¹ of the ridge-typeoptical waveguide 61. The first end of the optical waveguide 60 isconnected to the ridge-type optical waveguide 61 and the second end tothe rectangular optical waveguide 62. At the connecting point 601 of theoptical waveguide 60 and the ridge-type optical waveguide 61, the width1_(60a)=1_(601a) of the first layer 601 of the optical waveguide 60 isequal to the width 1_(h) of the ridge 61 ¹ of the ridge-type opticalwaveguide. At the connecting point 602 of the optical waveguide 60 andthe rectangular optical waveguide 62, the width 1_(60a)=1_(602a) of thefirst layer 60 ¹ of the optical waveguide 60 is equal to the width 1_(s)of the rectangular optical waveguide 62. At the connecting points 601,602 of the waveguides there is no material connecting area or similar,but the core elements consisting of the layers of different waveguidesare of the same material, and they connect to each other directly andseamlessly.

In the application example of FIG. 6, the width 1_(h) of the ridge 61 ¹of the ridge-type optical waveguide 61 is smaller than the width 1_(s)of the rectangular optical waveguide 62. Alternatively, depending on theapplication, the width 1_(h) of the ridge 61 ¹ is equal to or biggerthan the width 1_(s) of the rectangular optical waveguide 62. The width1_(60a) of the first layer of the optical waveguide 60 is thus arrangedto change from the first width 1_(601a), which is equal to the width1_(h) of the ridge 61 ¹ of the ridge-type optical waveguide at the firstconnecting point 601, to the second width 1_(602a), which is equal tothe width 1_(s) of the rectangular optical waveguide 62 at the secondconnecting point 602.

The height h_(60b) of the second layer 60 ² of the optical waveguide 60and at the same time the rise of the second step is equal to the heighth_(k) of the base part 61 ² of the ridge-type optical waveguide 61. Atthe connecting point 601 of the optical waveguide 60 and the ridge-typeoptical waveguide 62, the width 1_(60b)=1_(601b) of the second layer 60² of the optical waveguide is equal to the finite width 1_(61k) of thebase part 61 ² of the ridge-type optical waveguide. At the connectingpoint 602 of the optical waveguide 60 and the rectangular opticalwaveguide 61, the width 1_(60b)=1_(602b) of the second layer 60 ² of theoptical waveguide 60 is equal to the width 1_(s) of the rectangularoptical waveguide 62. The width 1_(61k) of the base part 61 ² of theridge-type optical waveguide 61 is in principle infinite, but inpractice, the second optical waveguide 60 and its second layer 60 ² areconnected to the base part 61 ² at the connecting point 601 in somesuitable finite width, which is so large that it has no significantinfluence on the activity of the optical waveguide. Preferably the width1_(61k) is the width 1_(h) of the ridge 61 ¹ multiplied by a constantfigure, which is calculated numerically.

The height h_(60a) of the first layer 60 ¹ of the optical waveguide 60of the invention is thus equal to the height h_(h) of the ridge 61 ¹ ofthe ridge-type optical waveguide, as again the height h_(60b) of thesecond layer 60 ² is equal to the height h_(k) of the base part 61 ² ofthe ridge-type optical waveguide. The height h_(s) of the rectangularoptical waveguide 62 again is the sum of the heights h_(60a) and h_(60b)of the layers 60 ¹, 60 ² of the second optical waveguide 60, i.e.h_(s)=h_(60a)+h_(60b)=h_(h)+h_(k).

The heights h_(60a), h_(60b) of the layers 60 ¹, 60 ² of the opticalwaveguide 60 according to the invention and thus the rises of the stepsdepend on the height dimensions h_(h), h_(k) of the ridge 61 ¹ and thebase part 61 ² of the ridge-type optical waveguide 61 and, respectively,on the height h_(s) of the rectangular optical waveguide 62. As isevident from above, the first layer 60 ¹ of the optical waveguide 60,i.e. the distance of the inner step pair is arranged to narrow (or towiden, respectively) in the direction of travel of light most preferablyuniformly and linearly from one width 1_(h) to second width 1_(s) (orvice versa, seen to the opposite direction of propagation of light).

The purpose of the optical waveguide 60 is to connect two opticalwaveguides 61, 62 of different shapes and at least partly with differentdimensions to each other. By applying the optical waveguide 60 of theinvention, this is achieved adiabatically in a desired way with as smalllight propagation losses as possible. In the embodiment example shown,the optical waveguide 60 and the optical waveguides 61, 62 connected byit are symmetrical in relation to their vertical middle plane.

The manufacture of the optical waveguide 60 and the ridge-type opticalwaveguide 61 and the rectangular optical waveguide 62 connected to it isperformed by utilising two, the first and second exposure masks 66, 67or a corresponding process pattern in two successive processing phases.In FIG. 5, the exposure masks 66, 67 are illustrated at a distance abovethe support 7 and the second optical waveguide 60 of the inventionarranged on it. The next more detailed description of the exposure masks66, 67 is based on the assumption of the use of the photolithographicpatterning described above. However, the same or similar masks can alsobe applied in connection of other patterning methods.

The width 1_(66a) of the first end 66 a of the first mask 66 correspondsto the width 1_(h) of the ridge 61 ¹ of the ridge-type optical waveguide61. From the mask point 601 a corresponding to the first connectingpoint 601, the first mask 66 widens towards the second end 66 b, and itswidth 1₆₆ is equal to the width 1_(60a) of the first layer 60 ¹ of theoptical waveguide 60 connecting the ridge-type optical waveguide 61 andthe rectangular optical waveguide 62 until the mask point 602 acorresponding to the second connecting point 602, from which onwards itin this embodiment widens further in a similar way as between the maskpoints 601 a, 602 a. To the direction shown after the second mask point602 a, i.e. to the direction of the rectangular optical waveguide 62,the width 1_(66b) of the first mask 66 is bigger than the width 1_(s) ofthe rectangular optical waveguide 62 to be processed, and its size is ofno significance as such; the rectangular optical waveguide 62 isrestricted to its final width 1_(s) with the help of the second exposuremask 67, as is evident from the following explanation.

The width 1_(67b) of the second end 67 b of the second exposure mask 67corresponds to the width 1_(s) of the rectangular optical waveguide 62.The second mask 67 widens from the mask point 602 b corresponding to thesecond connecting point 602 towards the first end 67 a, and its width1₆₇ is equal to the width 1_(60b) of the second layer 60 ² of theoptical waveguide 60 connecting the ridge-type optical waveguide 61 andthe rectangular optical waveguide 62 until the mask point 601 bcorresponding to the first connecting point 601, from which onwards itin this embodiment widens further in a similar way as between the maskpoints 601 b, 602 b. To the direction shown after the first mask point601 b, i.e. to the direction of the ridge-type optical waveguide 61, thewidth 1_(67a) of the second mask 67 is so much bigger than the width ofthe ridge-type optical waveguide 61 that its size has no significantinfluence on the activity of the optical waveguide. Thus it can be saidthat at the connecting point 601, the width 1_(67a) of the second mask67 corresponds to the finite width of the base part 61 ² of theridge-type optical waveguide 61.

Because the width 1₆₇ of the second mask 67 at the first connectingpoint 601 has no significant influence on the activity of the opticalwaveguide, the location of the first connecting point 601 is determinedonly on the basis of the mask point 601 a of the first mask 66corresponding to it. The width 1_(66b) of the first mask 66 from thesecond connecting point 602 towards the rectangular optical waveguide 62again has no influence on the activity of the optical waveguide as longas it is bigger than the width 1_(67b) of the second mask 67 at therespective place. The location of the second connecting point 602 issolely determined on the basis of the intersection points of the edgesof the masks 66, 67. In FIG. 6, the connecting point 602 is drawn tobecome congruent with the mask point 602 b for simplicity, but this neednot necessarily be the case. For example, the second mask 67 cancontinue to narrow for a short range onwards from the mask point 602 btowards the rectangular optical waveguide 62 so that also therectangular optical waveguide connected to the optical waveguide 60narrows respectively.

Because of the finite mask alignment accuracy, the masks 66, 67 or therespective process patterns can slightly move in relation to each otheras the optical waveguide is being manufactured. However, the operationof the optical waveguide 60 does not significantly change because ofsmall alignment errors, because on the basis of what has been saidabove, it is not necessary to align any mask points to each other in anabsolutely accurate manner. At most, the alignment errors slightly movethe connecting points 601 and 602 in the longitudinal direction of theoptical waveguide and make the optical waveguide 60 slightlyasymmetrical in relation to its longitudinal middle axis. By using maskpatterns that widen and narrow sufficiently flatly, the opticalwaveguide 60 stays sufficiently adiabatic also in this case.

The first and second exposure mask 66, 67 are used in the manufacture ofthe optical waveguide 60 and the related optical waveguides 61, 62. Inthe manufacture, in the etching step of the first processing cyclefollowing the use of the first mask 66 the etching is performed to thefirst depth h₁=h_(h)=h_(60a) so that the first layer 60 ¹ of the opticalwaveguide 60 and the ridge 61 ¹ of the first ridge-type opticalwaveguide 61 can be separated from the core layer 7 c on the support 7.The areas 64 a, 64 b removed in the first etching step are marked withbroken lines in FIGS. 7A, 7B and 7C. In the etching step of the secondprocessing cycle following the use of the second mask 67 the core layeris etched so that at the edges of the rectangular optical waveguide theetching extends through the whole core layer (to the depth h_(s)), andat the same time, the second layer 60 ² of the third optical waveguide60 is overetched through the remaining thickness h^(60b) of the corelayer. The second etching depth h₂ of the second etching phase isdifferent in different areas, due to overetching. The areas 65 a, 65 bremoved in the second etching step are marked with broken lines in FIGS.7B and 7C.

The optical waveguide 60 of the invention can also be realised by usinga preferable first varied manufacturing method, compared with theprevious method. In this case, the exposure masks 66 and 67 are used inreverse order in relation to the previous method, and in addition, acommon hard mask layer is used in connection of them. Thus, the removalof the hard mask and the adding of a new hard mask is passed between theprocessing cycles. In the etching step of the first processing cyclefollow-ing the use of the mask 67, the core layer is etched to the depthh_(60b), and in the etching step of the second processing cyclefollowing the use of the mask 66, the core layer is etched to the depthh_(60a). The latter etching step continues the etching of all areasetched in the first processing cycle (the edges of the rectangularoptical waveguide and the edges of the second layer of the opticalwaveguide 60) until the lower edge of the core layer and,simultaneously, it etches the areas 64 a and 64 b surrounding theridge-type optical waveguide and the areas between the adjacent steps 6;6 ^(1a), 6 ^(2a) and 6; 6 ^(1a), 6 ^(2a) to the depth h_(h)=h_(60a).With this method, the final result obtained will be the same structureas with the previous method, but in this way each etching phase etchesto the same depth in all areas, and thus such an overetching phase isavoided, in which the etching in some areas tends to pass the lower edgeof the core layer.

In relation to what is explained above, the optical waveguide 60 canalso be realised with the help of a second and third preferable variedmanufacturing method. In the second varied manufacturing method, thewidth 1_(60b) of the second layer of the optical waveguide 60 and thewidth 1₆₇ of the second mask 67 corresponding to it are arranged as aconstant, which is bigger than the width 1_(h) of the ridge of theridge-type optical waveguide 61, which is equal to the width 1_(s) ofthe rectangular optical waveguide 62. In this case, the width 1_(60a) ofthe first layer of the optical waveguide 60 and the width 166 of thefirst mask 66 corresponding to it are arranged to widen from the widthof the first mask point 601, i.e. the width 1_(h) of the ridge of theridge-type optical waveguide 61 towards the rectangular opticalwaveguide. In a third varied manufacturing method, the width 1_(60a) ofthe first layer of the optical waveguide 60 and the width 1₆₆ of thefirst mask 66 corresponding to it are arranged as a constant, which isequal to the width 1_(s) of the rectangular optical waveguide 62 at thesecond connecting point 602. In this case, the width 1_(60b) of thesecond layer of the optical waveguide 60 and the width 1₆₇ of the secondmask 67 corresponding to it are arranged to narrow from the width of thefirst mask point 601, i.e. the finite width 1_(61k) of the base part 61² of the ridge-type optical waveguide 61 towards the rectangular opticalwaveguide and slightly past the second mask point 602 so that no veryhigh requirements be set to the mask alignment. In the second and thirdvaried manufacturing method, the other dimensions of the opticalwaveguide 60 are kept the same and/or they are arranged to change, as isexplained above in connection of FIGS. 6, 7A, 7B and 7C.

A considerable advantage of the optical waveguide 60 and itsmanufacturing process is that no especially large alignment accuracy isneeded between the two process patterns used, as has been demonstratedabove. Only one process pattern, or a corresponding mask, determines thedimensions (i.e. especially the width and the length, but, in principle,also the height) of the optical waveguide 61, 62 to be connected to theoptical waveguide 60 of the invention. Because of the adiabatic propertyof the optical waveguide 60 and the small angles of crossing of theedges of the process patterns, small alignment errors do not largelyinfluence the transfer of light between the optical waveguides 61, 62 tobe connected.

The invention is not limited to concern the above presented embodimentexamples only, but many variations are possible within the inventionalidea determined by the claims.

-   /1/Hiroshi Nishihara, Masamitsu Haruna and Toshiaki Suhara: “Optical    integrated circuits”, McGraw-Hill Book Company, ISBN 0-07-046092-2,    1989-   /2/ Denis Donlagic and Brian Culshaw: “Propagation of the    fundamental mode in curved graded index multimode fiber and its    application in sensor systems”, Journal of Lightwave Technology, vol    18, pp. 334-342, 2000

1-7. (canceled)
 8. Waveguide, which is part of an integrated opticalcircuit, the waveguide being arranged onto a planar substrate and havinga core section propagating light to a certain direction, the directionof propagation, characterised in that the waveguide is a conversionwaveguide (60) between a ridge-type waveguide (61) and a strip waveguide(62), in which conversion waveguide the core section is made of the oneand same material so that the cross-section of the core sectiontransverse to the direction of propagation (z) of light is two-step (6;6 ^(1a), 62 ^(2a); 6 ^(1b), 6 ^(2b)) from both edges (60 a, 60 b), andin which conversion waveguide there are two layers (60 ¹, 60 ²) ofdifferent widths (1_(60a), 1_(60b)), the height (h_(60a)) of the firstlayer (60 ¹) being equal to the height of the ridge (61 ¹) of theridge-type waveguide (61), and the height (h_(60b)) of the second layer(60 ²) being equal to the height of the base part (61 ²) of theridge-type waveguide (61), and in which the sum of the heights (h_(60a),h_(60b)) of the layers (60 ¹, 60 ²) is equal to the height of the stripwaveguide (62), the widths of the two layers (601, 602) being arrangedto change uniformly between the waveguides to be connected for fittingthem together laterally.
 9. Waveguide according to claim 8,characterised in that the waveguide (60) is made of semiconductormaterial, especially silicon.
 10. Waveguide according to claim 9,characterised in that the waveguide (60) is made onto a SOI substrate.11. Waveguide according to claim 8, characterised in that the widths(1_(60a), 1_(60b)) of the layers (60 ¹, 60 ²) of the waveguide (60) arearranged to change linearly between the ridge of the ridge-typewaveguide (61) and the rectangular core section of the strip waveguide(62) of different widths for connecting them together with the help ofthe waveguide (60).
 12. Method for manufacturing an integrated opticalcircuit onto a substrate, characterised in that the waveguide is aconversion waveguide (60), which is manufactured between the ridge-typewaveguide (61) and the strip waveguide (62) onto such a substrate (7),on which there is a light-propagating core section (7 c), in whichmethod the core layer (7 c) is controllably thinned in two stages forforming two different steps on both sides of the conversion waveguide sothat different process patterns are utilised in both thinning stages,the edges of which determine the location of the edges of the steps ofthe waveguide on the substrate, so that the result obtained is awaveguide structure, which is two-step (6; 6 ^(1a), 6 ^(2a); 6 ^(1b), 6^(2b)) from both edges (60 a, 60 b) transverse to the direction ofpropagation of light, in which the conversion waveguide (60) is providedwith two layers (60 ¹, 60 ²) of different widths (1_(60a), 1_(60b)) sothat the height (h_(60a)) of the first layer (60 ¹) is arranged to beequal to the height of the ridge (61 ¹) of the ridge-type waveguide(61), and the height (h_(60b)) of the second layer (60 ²) is arranged tobe equal to the height of the base part (61 ²) of the ridge-typewaveguide (61), and in which the sum of the heights (h_(60a), h_(60b))of the layers (60 ¹, 60 ²) is arranged to be equal to the height of thestrip waveguide (62), and the widths of the two layers (601, 602) arearranged to change uniformly between the waveguides (61, 62) to beconnected for fitting them together in the lateral direction.
 13. Methodaccording to claim 12, characterised in that the waveguide (5) ismanufactured onto a suitable finished substrate (7), such as a SOI waferor similar.
 14. Method according to claim 12, characterised in that onecommon hard mask layer (9; 9 ¹) is used in it for providing at least twodifferent process patterns to the core layer (7 c) of the substrate. 15.Method according to claim 13, characterised in that one common hard masklayer (9; 9 ¹) is used in it for providing at least two differentprocess patterns to the core layer (7 c) of the substrate.